The big idea: A stem cell is an unspecialized cell that can do two special things:
it can divide to make more cells (self-renewal), and it can differentiate into one or more specialized cell types.
These two properties are exactly what doctors need to treat disease: stem cells can be grown in large numbers and then turned into the specific cells a patient has lost — for example new blood cells, nerve cells or skin cells.
- Stem cell
- An unspecialized cell that can keep dividing (self-renew) and can differentiate into one or more specialized cell types.
- Self-renewal
- The ability of a stem cell to divide by mitosis to produce more cells, including more stem cells, so the supply is not used up.
- Differentiation
- The process by which an unspecialized cell becomes a specialized cell with a particular structure and function.
- Therapeutic use
- Using stem cells to treat a disease or injury — usually by replacing cells that have been lost or damaged.
Two properties, one purpose: Whenever an exam asks why stem cells are useful, the answer comes back to the same two properties:
they divide (so you can make enough cells), and they differentiate (so those cells become the right specialized type).
Replace a lost cell type → you need cells that can multiply and then become that exact cell.
Many diseases happen because a particular specialized cell type is lost or damaged and the body cannot replace it.
Stem cells offer a treatment: they divide to make many new cells, then differentiate into the exact cell type that was lost, so the missing tissue can be replaced and its function restored.
| Property of a stem cell | What it means | Why it is useful in medicine |
|---|---|---|
| Self-renewal (divide repeatedly) | A stem cell divides by mitosis to make more stem cells, so the supply does not run out | A large number of new cells can be grown to replace many lost cells |
| Differentiation (become specialized) | A stem cell can turn into one or more specialized cell types (e.g. a nerve, blood or muscle cell) | The new cells can become the exact specialized cell type that the patient has lost |
The logic of stem-cell treatment: Think of it as a simple cause-and-effect chain:
a disease destroys a specialized cell → the patient is given stem cells → the stem cells divide to make many cells → those cells differentiate into the missing specialized cell → the tissue is replaced and its job is restored.
Example: in a disease that destroys the light-detecting cells of the eye, stem cells could divide and differentiate into new light-detecting cells to restore vision.
Where the stem cells come from: Stem cells for treatment come from two main sources, and the source matters:
Embryonic stem cells come from very early embryos and can become almost any cell type, so they are very flexible — but using an embryo raises ethical objections.
Adult (tissue) stem cells, such as those in bone marrow, can become only a few related cell types, but they raise far fewer ethical concerns because no embryo is used.
| Feature | Embryonic stem cells | Adult (tissue) stem cells |
|---|---|---|
| Where they come from | Very early embryos | Found in body tissues (e.g. bone marrow) |
| What they can become | Almost any cell type (pluripotent) — very flexible | Usually only a few related cell types (multipotent) — more limited |
| How easily obtained | Harder to obtain; the source is an early embryo | Can be taken from the patient or a donor |
| Main ethical concern | Using an early embryo, which would otherwise grow, raises ethical objections | Far fewer ethical objections — no embryo is used |
Why stem cells help (benefits)
- They divide to make many new cells
- They differentiate into the cell type that was lost
- They can replace damaged tissue the body cannot regrow
- They may treat diseases that currently have no cure
Why it raises concerns (issues)
- Embryonic stem cells come from an early embryo (ethical objection)
- Donor cells may be rejected by the patient's immune system
- A small risk the cells divide uncontrollably (tumours)
- Adult stem cells are more limited in what they can become
A memory hook: Divide and differentiate = treat. Stem cells multiply then become the missing cell.
And when you weigh up the treatment: benefit = replacing lost cells; issue = where the cells come from (an embryo raises the ethical objection).
Reading the result (data skill): Stem-cell results are a favourite for data questions. If a sample of stem cells is grown and the count of cells goes up and some become a named specialized cell type, the stem cells have done both jobs: they divided (numbers rose) and they differentiated (specialized cells appeared).
If numbers rise but no specialized cells appear, the cells divided but did not differentiate.
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How this is tested: On Paper 1 a short question often gives a disease in which a particular cell type is destroyed and asks why stem cells are suitable to treat it — the marks come from the two properties: stem cells divide to make many cells and differentiate into the exact cell type that was lost.
On Paper 2 you may have to discuss or evaluate stem-cell therapy, weighing the therapeutic benefit against the ethical issue of using embryonic stem cells.
On Paper 1B a data question may show how many cells were produced, and ask you to conclude whether the stem cells divided and differentiated as expected.
IB-style question — why stem cells suit this treatment
An inherited disease gradually destroys the light-detecting cells at the back of a patient's eye, causing blindness. Explain why stem cells are suitable to treat this disease. [3]
How to score all three marks
- Start with division (self-renewal). Stem cells can divide (by mitosis) to produce a large number of new cells, so there are enough cells to replace the many that were destroyed.
- Add differentiation. Stem cells can differentiate into specialized cell types, so they can become new light-detecting cells — the exact cell type the disease destroyed.
- Link to restoring function. The new light-detecting cells replace the lost ones, restoring the eye's ability to detect light and so improving vision. (Mark 1: divide / self-renew. Mark 2: differentiate into the specific lost cell type. Mark 3: replace cells / restore function.)
Final answer
Stem cells can divide to make many new cells and can differentiate into new light-detecting cells — the exact cell type destroyed — replacing the lost cells and restoring vision.
✓ Why this scores full marks: It uses both key properties — divide and differentiate — and ties them to the specific lost cell type, not a vague 'they help the eye'.
A common slip is naming only one property: 'they divide' alone earns one mark, but the differentiation into the correct specialized cell is what makes the treatment work.
| Therapeutic uses (benefits) | Ethical / practical issues (concerns) |
|---|---|
| Replace cells lost to disease or injury (e.g. blood, nerve or skin cells) | Embryonic stem cells are taken from an early embryo, which raises ethical objections |
| Repair tissue that the body cannot regrow on its own | Transplanted cells may be rejected by the patient's immune system if from a donor |
| Potentially treat conditions that currently have no cure | There is a small risk that the cells divide uncontrollably (tumour formation) |